Gas

The use of a low-Z gas is important for minimizing multiple Coulomb scattering contributions to the momentum resolution. Since low-Z gases have a smaller photo-electric cross section than argon-based gases, they have the additional advantage of reduced background from synchrotron radiation. We have selected a 50% helium-50% ethane gas mixture. This mixture has a long radiation length (640 m), and a drift velocity that saturates at 4 cm/$\mu$s at a relatively low electric field [32,35]. This is important for operating square-cell drift chambers because of large field non-uniformities inherent to their geometry. The use of a saturated gas makes calibrations simpler and helps to ensure reliable and stable performance. Even though the gas mixture has a low Z, a good $dE/dx$ resolution is provided by the large ethane component [36].
Several test chambers were constructed and beam tests were carried out. Figs. (a) and (b) show measured gas gain and drift velocity data for a 50% helium-50% ethane gas mixture [32].

Figure: (a) The measured gas gain and (b) drift velocity for a 50 % He and 50 % C$_2$H$_6$ gas mixture.